linux-kernel - Re: [PATCH][V2] backlight: sky81452: insure while loop does not allow negative array indexing

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Message-ID: <b0e21719-3a7c-099a-292d-c3fa65a84fe8@canonical.com>
Date:   Thu, 27 Feb 2020 15:10:43 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Gyungoh Yoo <jack.yoo@...worksinc.com>,
        Bryan Wu <cooloney@...il.com>, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][V2] backlight: sky81452: insure while loop does not allow
 negative array indexing

On 27/02/2020 11:46, Daniel Thompson wrote:
> On Wed, Feb 26, 2020 at 07:58:26PM +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> In the unlikely event that num_entry is zero, the while loop
>> pre-decrements num_entry to cause negative array indexing into the
>> array sources. Fix this by iterating only if num_entry >= 0.
>>
>> Addresses-Coverity: ("Out-of-bounds read")
>> Fixes: f705806c9f35 ("backlight: Add support Skyworks SKY81452 backlight driver")
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>
>> V2: fix typo in commit subject line
> 
> Isn't the correct spelling "ensure"?

It is. V1 is correct after all. Doh.

> 
> 
>> ---
>>  drivers/video/backlight/sky81452-backlight.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/video/backlight/sky81452-backlight.c b/drivers/video/backlight/sky81452-backlight.c
>> index 2355f00f5773..f456930ce78e 100644
>> --- a/drivers/video/backlight/sky81452-backlight.c
>> +++ b/drivers/video/backlight/sky81452-backlight.c
>> @@ -200,7 +200,7 @@ static struct sky81452_bl_platform_data *sky81452_bl_parse_dt(
>>  		}
>>  
>>  		pdata->enable = 0;
>> -		while (--num_entry)
>> +		while (--num_entry >= 0)
>>  			pdata->enable |= (1 << sources[num_entry]);
> 
> This look still looks buggy to me (so I'd second Walter's request to
> change it to a for loop). If the code genuinely does not contain a
> bug then it probably needs a prominent comment explaining why it is
> correct not to honour sources[0]!
> 
> 
> Daniel.
>